The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 9-199710
Japanese Patent Application No. 9-337104
Japanese Patent Application No. 10-67021
The present invention relates to a projection exposure apparatus employed to expose a pattern of an original such as a mask or a reticule (hereafter referred to as a mask) onto a photosensitive substrate such as a wafer during a photolithography process implemented during the fabrication of a semiconductor device such as an LSI, an image-capturing element such as a CCD, a liquid crystal display element, or a semiconductor device such as a thin film magnetic head, a projection exposure method utilizing this exposure aparatus, an optical cleaning method employed to clean the optical systems in the projection exposure apparatus and a method of fabricating a semiconductor device.
Keeping pace with the increasingly higher integration achieved for semiconductor devices, significant progress has been made in the area of projection exposure apparatuses employed during the photolithography process that is crucial in fabrication of semiconductor devices. The resolving power achieved by a projection optical system mounted at a projection exposure apparatus is expressed through the relational expression R=kxc3x97xcex/NA, known widely as Rayleigh""s formula. In this relational expression, R represents the resolving power of the projection optical system, xcex represents the wavelength of the exposing light, NA represents the numerical aperture at the projection optical system and k represents a constant which is determined by process-related factors as well as the resolving power of the resist.
The resolving power required of the projection optical system to support higher integration in the semiconductor device may be achieved by reducing the wavelength of the light from the exposing light source or by increasing the numerical aperture at the projection optical system as the relational expression above indicates. Thus, continuous efforts to achieve a higher NA value have been made. In recent years, exposure aparatuses that use a krypton fluoride excimer laser (KrF excimer laser) having an output wavelength of 248 nm as an exposing light source with the numerical aperture at 0.6 or higher achieved at the projection optical system have been put into practical use to enable exposure of extremely fine patterns of down to 0.25 xcexcm.
An argon fluoride excimer laser (ArF excimer laser) having an output wavelength of 193 nm has been attracting much attention recently as a light source to replace the krypton fluoride excimer laser. Since it is expected that by realizing an exposure apparatus using this argon fluoride excimer laser as the exposing light source, ultra-fine processing down to 0.18 xcexcmxcx9c0.13 xcexcm will be possible, concentrated efforts are being made in research and development.
Since there are at present only two materials, i.e., synthetic silica glass and calcium fluoride (fluorite), that may be used to constitute the lenses while achieving a satisfactory transmittance in the wavelength range of the output wavelength (193 nm) of the argon fluoride excimer laser, tireless efforts are being made to develop an optical material achieving sufficient transmittance and sufficient internal consistency to be used in this type of exposure apparatus. Currently, synthetic silica glass achieves an internal transmittance of 0.995/cm or higher, and calcium fluoride has reached a point at which the level of internal absorption can be disregarded.
The choice of material to constitute the anti-reflection film that is coated on the surface of the optical material, too, is extremely limited compared to the range of materials from which selection can be made to constitute an anti-reflection film used in the output wavelength range (248 nm) of the krypton fluoride excimer laser, and this also greatly restricts the degree of freedom afforded in design. However, thanks to the intense efforts made in development this problem, too, is being overcome. At present, the levels of losses at the individual lens surfaces (e.g., losses through the absorption of light by the coating, scattering of light, reflection at the interface of the coating and the optical material and reflection at the coating surface) have been lowered to 0.005 or less (light loss of 0.5% or less).
In wavelength ranges shorter than the wavelength of KrF excimer laser light, moisture and organic substances may adhere to the surfaces of the optical elements constituting the optical systems (illumination optical system, projection optical system) in the projection exposure apparatus, resulting in a reduction in transmittance of the optical systems. This problem is attributable to gas trapped within the space enclosed by a plurality of optical elements or moisture and organic substances generated from the inner walls of the lens barrel or the like supporting the optical systems becoming adhered to the surfaces of the optical systems.
FIG. 17 illustrates time-varying transmittance characteristics in an optical system. The figure presents the optical system transmittance, which represents the ratio of the illuminance of the exposing light between the laser light source and the mask and the illuminance of the exposing light on the wafer measured over specific intervals while irradiating pulse laser light continuously from the laser light source during the laser irradiation and is calculated for each measuring time point. The figure also presents a similar optical system transmittance during a time period in which the laser is stopped that is obtained by irradiating laser over appropriate time intervals and calculated at each laser irradiation. As FIG. 17 illustrates, after the start of laser light irradiation, the transmittance gradually increases and when a specific length of time has elapsed, a near-saturated state is achieved. This phenomenon of the optical system transmittance gradually recovering is due to moisture and organic substances adhering to the optical system surface being removed from the optical system surfaces by the laser irradiation. For this reason, it is conceivable to start an exposure operation after a near-saturated state of transmittance is achieved by irradiating exposing laser light over a specific period of time prior to the start of the exposure. However, this would cause a reduction in throughput. In addition, oscillation of the laser over a long period of time prior to the exposure would lead to poor durability of the laser light source and it is, therefore, not desirable. Furthermore, it is difficult to continuously irradiate exposing laser light at all times, including during replacement of the wafer or the mask.
A first object of the present invention is to provide a projection exposure method and a projection exposure apparatus that make it possible to sustain the illuminance of the exposing light on a photosensitive substrate at a target value at all times regardless of time-varying transmittance of the optical system.
A second object of the present invention is to provide a projection exposure method and a projection exposure apparatus that controls the accumulated light quantity (exposure dose) of the exposing light on a photosensitive substrate at a correct value that corresponds to the sensitivity of the photosensitive substrate even when the transmittances at the illumination optical system and the projection optical system change.
A third object of the present invention is to provide an optical cleaning method for cleaning the optical systems by predicting time-varying transmittance at the illumination optical system and the projection optical system.
A fourth object of the present invention is to provide a method of fabricating a semiconductor device that achieves an improvement in the yield by exposing a circuit pattern or the like on a semiconductor substrate by predicting time-varying transmittance at the illumination optical system and the projection optical system.
A fifth object of the present invention is to provide a projection exposure method and a projection exposure apparatus that makes it possible to achieve a correct exposure dose at a photosensitive substrate in correspondence to changes in the transmittance occurring at the illumination optical system and the projection optical system even when conditions under which the photosensitive substrate is exposed, conditions under which the mask is illuminated and the like are changed.
A sixth object of the present invention is to provide a projection exposure method and a projection exposure apparatus that prevent any fluctuation in the exposure dose occurring on a photosensitive substrate due to changes in the transmittance occurring at the illumination optical system and the projection optical system even when a change occurs in at least one of the following; the intensity distribution of the exposing light on the pupil surface of the projection optical system, i.e., the intensity distribution of a secondary light source within the illumination optical system (namely, the shape and size), the pattern on the mask to be transferred onto the photosensitive substrate and the numerical aperture at the projection optical system.
The present invention is applied in a projection exposure apparatus having an optical system that projects an image of a pattern illuminated by exposing light emitted by an exposing light source onto a photosensitive substrate with time-varying transmittance of the exposing light at the optical system and a projection exposure method employed in combination with the projection exposure apparatus. The objects described above are achieved by measuring the transmittance of the optical system with regard to light having a wavelength substantially equal to the wavelength of the exposing light at a plurality of time points, predicting of time-varying transmittance characteristics of the optical system based upon the plurality of transmittances thus measured and projecting the pattern onto the photosensitive substrate based upon the results of the prediction.
It is desirable to measure the transmittance using exposing light emitted from the exposing light source. The plurality of time points at which the transmittance is measured may be a time point before the pattern is projected onto the photosensitive substrate, i.e., a time point before the light having the wavelength which is substantially the same as that of the exposing light is irradiated on the optical system and a time point after the light having the wavelength which is substantially the same as that of the exposing light is irradiated on the optical system over a specific length of time. Alternatively, the plurality of time points at which the transmittance is measured may constitute a time point before the image of the pattern illuminated by the exposing light is projected onto the photosensitive substrate and a time point after the image of the pattern illuminated by the exposing light is projected onto the photosensitive substrate.
The plurality of time points at which the measurement is performed may constitute, for instance, a time point before the image of the pattern illuminated by the exposing light is projected onto a single photosensitive substrate and a time point after the image of the pattern illuminated by the exposing light is projected onto the single photosensitive substrate. Alternatively, the plurality of time points for performing the measurement may constitute a time point before the image of a pattern illuminated by the exposing light is projected onto a specific area on a photosensitive substrate and a time point after the image of the pattern illuminated by the exposing light is projected onto the specific area. In this case, the plurality of time points may be time points before and after exposure processing performed on the exposure area of a single chip or time points before and after exposure processing performed on an exposure area corresponding to one shot.
If the optical system includes an illumination optical system that illuminates a pattern with exposing light and a projection optical system that projects the image of a pattern illuminated by the illumination optical system onto a photosensitive substrate, it is desirable to predict time-varying transmittance only at the optical system at which the transmittance fluctuates.
According to the present invention, the intensity of the exposing light irradiated onto the photosensitive substrate can be adjusted based upon predicted time-varying transmittance characteristics. Alternatively, the accumulated light quantity of the exposing light irradiated onto the photosensitive substrate can be controlled at a correct value that corresponds to the sensitivity of the photosensitive substrate based upon calculated time-varying transmittance characteristics.
In the method for controlling the accumulated light quantity in which the photosensitive substrate is made to move relative to the exposing light from the mask to pass through the projection optical system in synchronization with the movement of the mask relative to the exposing light during the process of emitting a pulse beam of exposure light from an exposing light source and projecting a pattern formed on the mask onto the photosensitive substrate, the accumulated light quantity of the exposing light can be controlled at a correct value corresponding to the sensitivity of the photosensitive substrate by adjusting at least one of the intensity of the exposing light entering the photosensitive substrate, the width of the exposing light on the photosensitive substrate relative to the traveling direction in which the photosensitive substrate moves, the traveling speed of the photosensitive substrate moving relative to the traveling direction and the oscillation frequency of the exposing light source, based upon the time-varying transmittance characteristics.
In addition, the present invention is adopted in a method of fabricating a semiconductor device by using a projection exposure apparatus having an optical system that projects the image of a pattern illuminated by exposing light emitted from an exposing light source onto a photosensitive substrate with the time-varying transmittance of the exposing light at the optical system. The objects described above are achieved by measuring the transmittance of light having a wavelength that is substantially equal to the wavelength of the exposing light at the optical system at a plurality of time points, predicting of time-varying transmittance characteristics at the optical system based upon a plurality of measured transmittances and projecting the image of the pattern onto the photosensitive substrate based upon the results of the prediction.
Furthermore, the present invention is adopted in an optical cleaning method implemented to clean an optical system that projects the image of a pattern illuminated by exposing light from an exposing light source onto a photosensitive substrate provided in a projection exposure apparatus with the transmittance of the exposing light at the optical system changing over time. The objects described earlier are achieved by measuring the transmittance of light having a wavelength substantially equal to the wavelength of the exposing light at the optical system at a plurality of time points and optically cleaning the optical system while predicting the time-varying transmittance characteristics at the optical system based upon a plurality of measured transmittances.
The projection exposure apparatus according to the present invention may comprise a mask illuminance detector that detects an illuminance of an exposing light irradiated on a mask from an exposing light source, a substrate illuminance detector that detects the illuminance of the exposing light on a photosensitive substrate, a means for prediction that predicts time-varying transmittance characteristics of the exposing light at the projection optical system by calculating the ratio of the illuminance of the exposing light irradiated on the mask detected by the mask illuminance detector and the illuminance of the exposing light irradiated on the substrate detected by the substrate illuminance detector a plurality of times and a control device that adjusts the accumulated light quantity of the exposing light entering the photosensitive substrate based upon the predicted time-varying characteristics and the ratio of the two illuminances.
If the exposing light source is constituted of a pulsed light source, the control device may adjust at least, either the intensity of pulsed exposing light irradiated onto the photosensitive substrate or the number of pulses to ensure that the accumulated light quantity of the exposing light irradiated onto the photosensitive substrate achieves a correct value that corresponds to the type of the photosensitive substrate based upon the predicted time-varying characteristics and the ratio of the two illuminances.
The projection exposure method according to the present invention comprises a step in which time-varying transmittance characteristics of the exposing light at, at least, either an illumination optical system or a projection optical system are predicted by calculating the ratio of the illuminance of the exposing light emitted by an exposing light source and the illuminance of the exposing light on a photosensitive substrate a plurality of times and a step in which, at least, either the intensity of the pulsed exposing light entering the photosensitive substrate or the number of pulses is adjusted based upon the ratio of the iilluminance of the exposing light emitted by the exposing light source and the illuminance of the exposing light on the photosensitive substrate and the predicted time-varying transmittance characteristics.
The present invention is adopted in an exposure method implemented in a projection exposure apparatus having an illumination optical system that illuminates a mask on which a specific pattern is formed with exposing light emitted by an exposing light source and a projection optical system that projects the image of the pattern on the mask illuminated by the illumination optical system onto a photosensitive substrate, with the transmittance of the exposing light at, at least, either the illumination optical system or the projection optical system changing over time. The objects described earlier are achieved by adjusting the intensity of the exposing light irradiated onto the photosensitive substrate based upon the ratio of the illuminance of the exposing light emitted by the exposing light source and the illuminance of the exposing light on the photosensitive substrate and the time-varying characteristics of the exposing light transmittance at, at least, either the illumination optical system or the projection optical system. Or, if the exposing light source is constituted of a pulsed light source, the objects described earlier are achieved by adjusting at least, either the intensity of the exposing light entering the photosensitive substrate or the number of pulses.
In a projection exposure apparatus in which the exposing light transmittance changes over time at its projection optical system alone among the illumination optical system and the projection optical system, the objects described above can be achieved by adjusting the accumulated light quantity of the exposing light entering the photosensitive substrate based upon the ratio of the illuminance of the exposing light emitted from the exposing light source and the illuminance of the exposing light on the photosensitive substrate and also based upon the time-varying characteristics of the exposing light transmittance at the projection optical system.
If the exposing light transmittance at the illumination optical system, too, changes over time, the objects described earlier can be achieved by adjusting the intensity of the exposing light emitted from the exposing light source based upon the time-varying characteristics of the exposing light transmittance in the entire optical system comprising the illumination optical system and the projection optical system and the ratio of the illuminances noted-above.
If the exposing light is a pulse beam, the accumulated light quantity of the exposing light can be controlled at a correct value that corresponds to the sensitivity of the photosensitive substrate by adjusting, at least, either the intensity of the exposing light entering the photosensitive substrate or the number of pulse beams of the exposing light irradiated on a given spot on the photosensitive substrate.
The exposure method described above may be adopted on an exposure apparatus that transfers a pattern onto a photosensitive substrate by causing the substrate to travel relative to the exposing light from the mask passing through the projection optical system in synchronization with the movement of the mask relative to the exposing light. In such a scan-type exposure apparatus, the exposing light may be a pulse beam. When the exposing light is a pulse beam, the accumulated light quantity of the exposing light is controlled at a correct value that corresponds to the sensitivity of the photosensitive substrate by adjusting at least one of: the intensity of the exposing light entering the photosensitive substrate, the width of the exposing light on the photosensitive substrate relative to the traveling direction in which the photosensitive substrate moves, the traveling speed at which the photosensitive substrate moves in the traveling direction and the oscillation frequency of the exposing light source.
The present invention is adopted in a projection exposure apparatus having an illumination optical system that illuminates a mask at which a specific pattern is formed with exposing light emitted by an exposing light source and a projection optical system that projects the image of the pattern at the mask illuminated by the illumination optical system onto a photosensitive substrate with the exposure light transmittance at the projection optical system changing over time. The objects described earlier are achieved by providing a mask illuminance detector that detects the illuminance of the exposing light irradiated on the mask from the exposing light source, a substrate illuminance detector that detects the illuminance of the exposing light on the photosensitive substrate, a storage device that stores in memory time-varying characteristics of the exposing light transmittance at the projection optical system and a control device that adjusts the accumulated intensity of the exposing light entering the photosensitive substrate based upon the ratio of the illuminance of the exposing light irradiated on the mask detected by the mask illuminance detector and the illuminance of the exposing light irradiated onto the photosensitive substrate detected by the substrate illuminance detector and the time-varying characteristics stored in the storage device. In addition, the control device may adjust at least, either the intensity or the number of the exposing light irradiated onto the photosensitive substrate to ensure that the accumulated light quantity of the exposing light irradiated onto the photosensitive substrate achieves a correct value that corresponds to the photosensitive substrate.
If the exposing light transmittance at the illumination optical system also changes over time in this projection exposure apparatus, it is desirable to store in memory the time-varying characteristics of the exposing light transmittance in the entire optical system comprising the illumination optical system and the projection system in the storage device. The storage device is capable of storing a plurality of sets of time-varying transmittance characteristics of the exposing light in correspondence to various exposure conditions. If a given set of exposure conditions does not match the exposure conditions stored in memory in the storage device, the transmittance is calculated through an interpolation operation performed on the time-varying characteristics stored in memory. The exposure conditions in this context refer to the illumination conditions of the illumination optical system (e.g., the diameter of the variable aperture stop at the illumination system, a modified illumination or a normal illumination etc.), the type of the mask and the numerical aperture at the projection optical system.
It is to be noted that if the exposing light is continuous light, either the intensity of the exposing light on the photosensitive substrate or the length of the irradiation period may be adjusted, or both may be adjusted. If the exposing light is pulsed light, either the pulse intensity of the exposing light on the photosensitive substrate or the number of pulses may be adjusted, or both may be adjusted. When exposure is achieved by scanning exposing light onto an exposure area on the photosensitive substrate that corresponds to the pattern area of the mask, at least one of: the intensity of the exposing light, the width of the exposing light along the scanning direction, the scanning speed of the substrate along the scanning direction and the oscillation frequency of the light source may be adjusted.
As explained above, according to the present invention, exposure performed on a photosensitive substrate is controlled based upon predicted time-varying transmittance characteristics of the exposing light calculated for, or based upon time-varying transmittance characteristics of the exposing light corresponding to various exposure conditions that are stored in advance. Thus, even when the transmittance at the illumination optical system or the projection optical system fluctuates during an exposure operation or while the apparatus is in a stopped state, the photosensitive substrate can be exposed correctly. For instance, the illuminance on the photosensitive substrate can be controlled at a correct value, or the accumulated light quantity (exposure dose) of the exposing light on the photosensitive substrate can be controlled at a correct value that corresponds to the sensitivity of the photosensitive substrate at all times.
In addition, according to the present invention, even when conditions under which the photosensitive substrate is exposed, conditions under which the mask is illuminated and the like are changed, or even when, at least one of: the intensity distribution of the exposing light on the pupil surface at the projection optical system, i.e., the intensity distribution of a secondary light source within the illumination optical system (the shape and size), the pattern on the mask to be transferred on the photosensitive substrate and the numerical aperture at the projection optical system, is changed, fluctuations in the exposure dose on the photosensitive substrate due to changes occurring in the transmittances at the illumination optical system and the projection optical system can be prevented from occurring by calculating the predicted time-varying transmittance characteristics that correspond to the specific changes or by storing in memory in advance time-varying transmittance characteristics that correspond the specific changes.
Furthermore, the predicted time-varying transmittance characteristics may be calculated by irradiating light having a wavelength substantially equal to the wavelength of the exposing light prior to the actual exposure operation so that the predicted time-varying transmittance characteristics can be calculated concurrently during optical cleaning performed prior to the exposure processing to prevent any reduction in throughput.
Moreover, according to the present invention, in which a semiconductor device is fabricated by calculating predicted time-varying transmittance characteristics and controlling the exposure operation based upon the predicted characteristics, an improvement is achieved in the production yield of semiconductor devices.